Given that your Mac at home is Mac 1, your Mac at work is Mac 2, your home's external IP address (how you're seen on internet) is X.X.X.X and your home's local IP address (LAN) is Y.Y.Y.Y, the DIY-way to do it is the following:
On your Mac 1 go in System Preferences -> Sharing and turn on Screen Sharing.
Open the port 5900 on your router pointing to your Mac's local address (Y.Y.Y.Y). This is the port used for the Screen Sharing service. Every router has its interface: you've to check yours and set it accordingly.
To reach your Mac 1, from Mac 2 choose Finder -> Go -> Connect to Server (or ⌘K in Finder) and type vnc://X.X.X.X. Fill the requested username and password and the Screen Sharing should start.
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For security reasons it's preferred to add a SSH tunneling to the solution described above. In this case on Mac 1 activate System Preferences -> Romote Login as well and open the port 22 on your router instead of 5900.
On Mac 2 open Terminal and type ssh -C -L 8000:127.0.0.1:5900 username@X.X.X.X where "username" is the Mac 1's username. Press Enter and fill the requested password.
The SSH tunneling session will start. Keep the Terminal open.
Choose Finder -> Go -> Connect to Server (or ⌘K in Finder) and type vnc://127.0.0.1:8000. Fill the requested username and password and the Screen Sharing should start.
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Easy solutions are available via apps like Screens 4 or Remotix 5.
